Again, this code works if the main application and the loaded application are in the same applicationDomain, and through debugging it appears the casted data (swfloader.content) is the same class _LoadedSWF_mx_managers_SystemManager but no error...

Hi eprevot, this to do with application domain sandboxing to prevent malicious code from excuting with the same authority as the host application. I would suggest you have a read through The Marshall Plan as this should provide you with the answer you are looking for.